1. Field of the Invention
The present invention relates to a Peltier device and a control circuit thereof, and more particularly to the structure of a Peltier device for fixing an operating temperature of a semiconductor laser and a temperature control of the Peltier device.
2. Description of the Related Art
Presently, in order to fix a temperature of a laser diode (LD) device, a Peltier device is used in a laser diode module for high speed optical communication, e.g., 2.4 G bit/sec, etc.
For example, a laser diode that emits a light beam is mounted on the top of a Peltier device, and the Peltier device performs a heating or cooling operation so as to fix the operating temperature of the laser diode.
A Peltier device has a plurality of P-type semiconductor devices and N-type semiconductor devices that are alternately arranged in relation to each other. The top portions of the P-type and N-type semiconductor devices are combined with a ceramic cooling plane, and the bottom portions thereof are combined with a ceramic radiating plane.
In the cooling plane, a current output from a DC (direct current) power supply flows from the N-type semiconductor devices to the P-type semiconductor devices. Considering the above current flow as a flow of electrons, electrons flow from a P-type lower energy level to a N-type higher energy level.
Consequently, the temperature at places combining the top portions of the P-type and N-type semiconductor devices with the cooling plane decreases by absorbing the surrounding thermal energy and thereby a temperature of the cooling plane decreases.
Conversely, in the radiating plane, the current flows from the P-type semiconductor devices to the N-type semiconductor devices. Thus, the radiating plane is heated up by effects opposed to the above-mentioned.
As described above, a heating operation or cooling operation by a Peltier device can be controlled by varying a quantity of current flowing therethrough or altering the direction of the current. A Peltier control circuit controls a quantity or the direction of a current flowing through a Peltier device by detecting the temperature of a device, for example a laser diode in this example, combined with the Peltier device so that the temperature of the device is kept constant.
A conventional Peltier control circuit has a push-pull type driving circuit consisting of two pairs of transistors, arranged in a Darlington configuration. The driving circuit heats a Peltier device by outputting a current thereto. Conversely, the driving circuit cools a Peltier device by inputting current. Also, in order to prevent an over-current that could destroy a Peltier device or cause thermal runaway, the driving circuit has a current limiting circuit that limits a quantity of a current flowing through a Peltier device by limiting a base current applied to the output transistors. Conventionally, a resistor is used for the current limiting circuit. In order to prevent such thermal runaway, etc., the driving circuit has a dead zone in which the output transistors are turned OFF close to an alternating point between heating and cooling operations.
The control of alternation between heating and cooling operations is performed by comparing a voltage detected by a temperature sensor, which measures the operating temperature of a Peltier device, with a reference voltage a value of which is set within the dead zone. Therefore, in this case, it is impossible to control a temperature of a Peltier device when the voltage detected by the temperature sensor is within the dead zone.
The range of a dead zone is determined by the characteristics of a transistor (V.sub.BE). Therefore, in the prior art there is a problem in that the difference in temperature between a sensing temperature and a control temperature occurs within a dead zone. Further, there is another problem in that the circuit loop stability for feedback control of the temperature of a Peltier device decreases in the dead zone due to the repetition between heating and cooling operations so that a loop oscillation may be caused.
Also, there is a problem in that when narrowing the dead zone by making the value of the current limiting resistor small, a loop oscillation alternately repeating between heating and cooling operations is more likely, and further, an over-current flows when supplying power.
Further, there is a problem in that an oscillation phenomena that is different from the above circuit loop oscillation occurs due to a structural factor of a conventional Peltier device. Namely, the combination of inductive elements of lead wires connected to an input and output of a laser diode and a capacitive element due to the structure of a Peltier device as described above, produces a resonant frequency point.
Therefore, conventionally, when using a laser diode module for high-speed optical communication, e.g., 2.4 G bit/sec, etc., there is a problem in that the prescribed oscillation phenomena occurs in the frequency range of optical communication.